The current state of the art in the carbon-nanotube (CNT) community is to grow aligned nanotubes on a crystalline quartz substrate. The tubes are then transferred to an oxidized silicon wafer using some variation of a transfer process. Source and drain contacts are typically defined using either i-line or e-beam lithographic techniques while the metal is deposited using an e-beam evaporator. Metallic tube removal is performed using the global silicon wafer as a backgate. This accomplishes the metallic tube removal process, but the global backgate incorporates an enormous parasitic capacitance. This extra gate capacitance prevents these devices from operating at high frequency.
When grown using a standard high-temperature chemical vapor deposition (CVD) based process, one-third (⅓) of the nucleating CNTs grow as metallic tubes while two-thirds (⅔) grow as semi-conducting tubes. Because the metallic tubes have conductance without having any trans-conductance, they are very undesirable from a device standpoint. Removing these tubes has been and is still a very active area of CNT research. The standard method of removing metallic tubes is to place or grow the tubes on a thin oxide coating a conductive wafer (e.g., a heavily doped silicon wafer). The conductive wafer is then used as a global backgate to deplete the semi-conducting tubes before passing a large current through the metallic tubes. In an oxygen environment, this high current will oxidize the metallic tubes, leaving behind most of the depleted semi-conducting tubes.